1. Field of the Invention
This invention, in its preferred form, relates to apparatus and methods for generating and directing pulses of a laser beam onto a work piece in a manner to ensure a strong weld of deep penetration. More particularly, this invention relates to apparatus and methods for welding a work piece by precisely controlling the lasing energy and in particular controlling the number of laser pulses imparted to the work piece. In an illustrative embodiment of this invention, the work piece takes the form of a nuclear fuel assembly, wherein its grid spacers are welded together at their points of intersection.
2. Description of the Prior Art
The precision laser welding apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assemble 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2E. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grid strap. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grids straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22. Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34 and 40 is carried out. The precision laser welding apparatus of this invention not only controls the various parameters of generating the laser in terms of the pulse width, the pulse height of each laser pulse, and the number of pulses to be applied to each weld, but also controls the sequential positioning of the fuel rod grids 16 with respect to the laser beam. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The manner of assembling the inner grid straps 20 to each other as well as to the outer grid straps 22 is shown in FIG. 2D. Each of the inner grid straps 20 includes a plurality of complementary slots 52. An upper grid strap 20a has a downwardly projecting slot 52a, whereas a lower grid strap 20b has a plurality of upwardly oriented slots 52b of a configuration and size to be received within a corresponding slot 52a of the inner grid strap 20a. At each end of the inner grid strap 20, there is disposed a pair of the tabs 26 to be disposed within corresponding slots 28 of an outer grid strap 22.
As will be explained in detail later, the inner grid straps 20 are welded to each other by the intersect welds 32 as formed of projection tabs 48 and tab portions 50a and 50b. More specifically, a projection tab 48 is disposed between a corresponding set of tab portions 50a and 50b when the inner grid straps 20a and 20b are assembled together. Upon the application of a laser beam to the tab 48 and tab portions 50a and 50b, an intersect weld 32 is formed that is rigidly strong and free of contamination in accordance with the teachings of this invention. Further, each end of an outer grid strap 22 has a corner tab 54. As shown in FIG. 2D, the outer grid straps 22c and 22b have respectively corner tabs 54b and 54c that overlap each other and are seam welded together to form the corner seam weld 30.
The vanes 42 project, as seen in FIGS. 2C and 2E, from a vane side of the fuel rod grid 16 to enhance the turbulence of the water passing over the nuclear fuel rods 18. Further, as illustrated particularly in FIG. 2C, the guide sleeves 36 are aligned with cells formed by the inner grid straps 20 that are free of either a resilient finger 44 or spacing finger 46, to thereby permit the free movement of the control rod through the cell and through the guide sleeve 36.
U.S. Pat. No. 3,966,550 of Foulds et al., and U.S. Pat. No. 3,791,466 of Patterson et al., assigned to the assignee of this invention, disclose similarly configured fuel rod grids of the prior art. Each of these patents discloses a fuel rod grid wherein the inner and outer grid straps are made of a suitable metallic alloy such as Inconel, and the above identified interconnections are effected by furnace brazing. However, the zirconium alloy Zircaloy is known to have the desirable characteristic of a low neutron absorption cross section which allows for more efficient use of the nuclear fuel in the utility operation and therefore allows for a longer elapsed time between refueling by the replacement of the nuclear fuel bundle assemblies. In particular, fuel rod grids made of Zircaloy have a lower absorption rate of the neutrons generated by the fuel rods than that absorption rate of straps made with Inconel. The making of the grid straps of Zircaloy requires at least several changes in the assembly of the fuel rod grids. First, it is necessary to make the slots, whereby the inner grid straps may intersect with each other, of looser tolerances in that grid straps made of Zircaloy do not permit a force fitting thereof, i.e. to be hammered into position, but rather require controlled fit-up to allow "push-fits" of the intersecting grid straps. In addition, Zircaloy grid straps may not be brazed in that heating Zircaloy to a temperature sufficient to melt the brazing alloy would anneal the Zircaloy, resulting in a loss of mechanical strength.
Prior to the selection of a particular method of welding, several different methods of welding volatile materials such as Zircaloy were investigated including continuous welding with a CO.sub.2 laser, pulsed emission of a Nd:YAG laser, gas tungsten arc welding and electron beam welding. A pulsed electron beam is capable of power densities of up to 10.sup.9 watts/square centimeter with pulse widths in the micro-second and low milli-second range. However, welding with an electron beam is typically carried out in a vacuum environment which is relatively expensive to build and requires a relatively long time to establish the desired degree of vacuum therein, thus slowing down the manufacture of the fuel rod grids. Further, it is necessary to obtain relative movement of the work piece, e.g. the fuel rod grids, in three dimensions with respect to the electron beam which would require a very complex grid positioning system. The use of a continuous electron beam provides relatively low levels of power (in the order of 200 watts) requiring relatively long welding times and providing very shallow weld penetrations. The use of a gas tungsten arc was also considered and proved to be unacceptable for providing a sequence of welds in that after a given number, of welds, e.g. 25, the arc electrodes require sharpening to provide the desired fine arc to produce numerous well defined welds and to avoid damaging adjacent grid straps or vanes of the fuel rod grids. Two types of lasers are commonly used for welding applications: (1) the solid state Nd:YAG laser, as contemplated by this invention, which uses a crystal rod of neodynium doped yttrium-aluminum-garnet and (2) the CO.sub.2 laser, which uses a mixture of CO.sup.2 --N.sub.2 --He as the lasing medium. An inherent advantage of the Nd:YAG laser is that its emission is in the order of 1.06 micron wave lengths, where glass is transparent to its laser emission. This characteristic permits the use of a coaxial microscope which uses the same optic elements for both optical viewing and laser focusing. Further, a pulsed Nd:YAG laser is capable of 400 watts of average power, of a pulse frequency of up to 200 pulses per second and of a peak power in excess of 8000 watts for up to 7 milli-seconds. Such high peak power permits the Nd:YAG laser to produce welds of relatively deep penetration, thus insuring the structural security of welded straps of the nuclear fuel rod grids. Such lasers may be operated either from a "cold start" with its shutter remaining open, whereby the weld time is determined by the length of time the power is applied to its flash lamps. Such a method of welding is not particularly applicable to a series of relatively rapid welds due to the laser rod warm-up time for each weld in the order of 0.8 seconds. Further, optical path length changes occur until a condition of thermal equilibrium is attained within the laser rod. A second method of operation, as contemplated by this invention, of the Nd:YAG laser permits the continuous pulse operation of the laser while using its shutter to "pick off" a fixed number of pulses, thus eliminating the effects of laser rod warm-up and ensuring a uniformity of welds even though a great number of such welds are being effected.
U.S. Pat. No. 3,555,239 of Kerth is an early example of a large body of prior art disclosing automated laser welding apparatus in which the position of the work piece, as well as the welding process, is controlled by a digital computer. Kerth shows the control of laser beams while controlling the work piece as it is moved from side to side along an X axis, horizontally forward and backward along a Y axis and vertically up and down along a Z axis. Typically, pulse driven motors are energized by the digital computer to move the work piece rectilinearly along a selected axis. In addition, the welding is carried out within a controlled atmosphere and, in particular, the pressure and flow of gas into the welding chamber is controlled by the digital computer. Further, a counter is used to count pulses, whereby the number of laser pulses applied to the work piece may likewise be controlled.
U.S. Pat. No. 4,263,495 of Fujita et al. discloses the use of a photodiode for receiving radiation from a weld site, i.e. the seam between two optical fibers, for measuring the power of the laser beam directed thereto during the course of the welding.
U.S. Pat. No. 4,088,890 of Waters discloses a programmable controller for controlling laser emission and, in particular, the control of a high beam shutter whereby the desired quantity of laser emission is directed onto the work piece. This patent also discloses the rectilinear movement of a carriage carrying the work piece along a vertical axis, whereby the work piece is successfully fully brought to a position where a laser weld is made. There is also disclosed the effecting of a seam weld, whereby the work piece is rotated while the laser beam is directed at a seam between two pieces to be welded together.
In particular, the Waters' patent discloses the use of a first set of lug sensors each disposed at the same level with respect to the vertical axis and 45.degree. up stream of the laser welding head for defining the weld zone. In particular, the lug sensors provide a high output indicating the commencement of the welding zone and, in particular, a seam between two abutting lugs. It is understood that after the lugs are rotated past, the lug sensors detect a loss of laser reflection to indicate the passing of the trailing edges of the lugs. The rotation of the leading and trailing edges of the lugs past the lug sensors controls a shutter to direct laser emission onto the seam between the two adjacent lugs. A further set of level sensors are provided, whereby the work piece as carried by the carriage rectilinearly along the vertical axis is controlled to dispose the seam between adjacent lugs at a vertical position aligned with respect to the laser head. In particular, the level sensors detect a substantial reduction of the output of the reflected radiation as the seam between two lugs passes the level sensors, whereby the rectilinear movement of the carriage and its work piece along the vertical axis is stopped.
U.S. Pat. No. 3,858,025 of Sidbeck et al. discloses the use of two infrared sensors for assisting in the positioning of a work piece with respect to a welding tool in the form of a welding electrode. The work piece takes the form of two thin sheets disposed with respect to each other in a "T" relationship with the welding electrode being positioned to form a seam weld at the point of intersection between the two thin sheets. In particular, a pair of infrared sensors are disposed upon either side of the vertically disposed sheet of the work piece to sense any unbalance in the welding puddle heat. A suitable circuit is connected to the output of the infrared sensors to detect the unbalance and to drive a reversible motor in accordance with the sensed unbalanced to reposition the welding electrode.
The above-described sensing systems of the prior art are primarily related to seam welding, wherein the welding of a seam between two work pieces is controlled. In particular, the prior art has dealt with the initiating and terminating of a seam weld as well as the alignment of the seam with respect to a laser head. By contrast, this invention relates to the accurate alignment of a machining site in the illustrative form of a spot weld with respect to the laser path. As described above, the work piece to be welded takes the form of the fuel rod grid 16, wherein the machining sites comprise the points 24 of intersection between the inner grid straps 20, whereat intersect welds 32 are to be performed. As will be explained, if any of the points 24 of intersection is misaligned with respect to the laser path, the intersecting inner straps 20 will not be welded with respect to each other thus providing a defective grid. In addition, it is necessary to not only align one point 24 of intersection, but a plurality of such points 24. In this regard, means in the form of a pair of X and Y positioning tables are used to receive and to incrementally move the fuel rod grid 16 to each of a sequence of positions, whereby each of the points 24 of intersection is accurately aligned with respect to the laser path.